The recovery of tin from aqueous solutions using tri-n-octylphosphine oxide (TOPO) as a solvent extraction reagent is well-known.
White and Ross in "Separations by Solvent Extraction with Tri-n-Octylphosphine Oxide", National Research Council, Subcommittee on Radiochemistry, Technical Information Center, Oak Ridge National Laboratory, 1961, report that tin is readily extracted from dilute acid chloride solutions by using TOPO. There is no suggestion in this publication, however, that tin can be selectively and efficiently extracted from acidic solutions to the exclusion of other metal cations by modifying, e.g., by branching, the alkyl substituents in the phosphine oxide. Moreover, it is stated in this publication on page 4 that no suitable method has been developed for the stripping of tin (IV).
Ross and White, "Analytical Chemistry," v.33(3), March 1961, pp. 424-427, also disclose the use of tris (2-ethylhexyl) phosphine oxide (TEHPO) to extract tin from acidic chloride-sulfate solutions. It is stated that ten additional elements are extracted with TEHPO. Among these elements, antimony, gallium, iron and molybdenum are extracted in an amount sufficient to interfere with the determination of tin although back-washing can eliminate or reduce these interferences. This article does not teach that the extraction of tin can be selectively enhanced to the exclusion of other metals by using phosphine oxides substituted with branched alkyl substituents having at least two branches per saturated alkyl.
Heddur and Khopkar, "Analyst," November 1984, Vol. 109, pp. 1493-1495, disclose that tin is separated from 2 M hydrochloric acid by extraction chromotography using TOPO as the stationary phase on a column of silica gel. They further disclose that the separation of tin from a larger number of elements in binary or multi-component mixtures can be carried out by exploiting the differences in their extractability with TOPO at various acidities of hydrochloric acid, e.g., 0.1 to 3 molar. There is no teaching or suggestion, however, that the selective extraction of tin can be enhanced by increasing the degree of branching in the phosphine oxide substituents.
Phosphine oxides have been proposed to extract halogenated hydrocarbons, alcohols, and rhenium metal. See, e.g., U.S. Pat. No. 4,470,909; U.S. Pat. No. 4,544,779; and European Pat. Appl. No. 113,912.
In U.S. Pat. No. 4,470,909, Bright discloses a method for the extraction of halogenated hydrocarbons, e.g., polychlorinated biphenyls (PCBs) from aqueous solutions using as the extractant phosphine oxides on inert solid supports. Such phosphine oxides can be linear or branched, e.g., tris (2,4,4-trimethylpentyl) phosphine oxide. Although the solvent systems taught by Bright are effective in the removal of halogenated hydrocarbons, there is no suggestion that they could be used to extract tin from acidic solutions bearing other metal cations.
In U.S. Pat. No. 4,544,779, Bright discloses a method for the recovery of lower alcohols from aqueous solutions using tertiary alkyl phosphine oxides in which the alkyl group is 4-18 carbon atoms. The use of symmetrical branched and unbranched phosphine oxides is disclosed, as is the use of unbranched and branched unsymmetrical phosphine oxides. The use of combinations of symmmetrical branched phosphine oxides with unsymmetrical unbranched or branched phosphine oxides is not disclosed. The compound n-hexyl-bis(2,4,4-trimethylpentyl)phosphine oxide is disclosed but no suggestion is made that such a compound can or should be combined with other phosphine oxides, especially symmetrical branched tertiary phosphine oxides, to extract tin from aqueous acid solutions.
In European Patent Application No. 113,912, published July 25, 1984, Bright discloses a process for the selective extraction of rhenium metal from aqueous sulfuric acid solutions using branched symmetrical tertiary phosphine oxide extractant compounds, e.g., tris(2,4,4-trimethylpentyl) phosphine oxide. However, no mention is made as to its usefulness for the selective extraction of tin or other Group IVA metals from acidic leach solutions, either alone or in further combination with unsymmetrical phosphine oxides.
It has now been unexpectedly discovered that solvents comprising branched tertiary phosphine oxides with at least two branches per saturated alkyl group are quite useful, in fact, more useful than conventional TOPO solvents, in selectively extracting tin from acidic leach solutions. These branched tertiary phosphine oxides can be employed alone or preferably in further combination with unsymmetrical tertiary phosphine oxides, phase modifying agents, diluents, etc., and are applicable to a great number of acidic leach solutions, e.g., scrap or ore, concentrates, and plating baths, which contain other metal cations, such as zinc(III), iron(II), iron(III), copper(II) and lead(II).
It has also been unexpectedly found that the tin-loaded solvents can be substantially stripped of tin by contacting with an effective amount of a stripping agent comprising an alkali earth metal hydroxide.